Polyimides find extensive use in electronic applications where they are useful in forming dielectric films on semiconductors devices. They are particularly useful as protective coatings for semiconductor devices.
It is well known in the polymer art to make thermally stable all-aromatic polyimides by the condensation polymerization of dianhydrides and diamines to form polyamic acid. Such polyimide precursors are disclosed inter alia in U.S. Pat. No. 3,179,634 to Edwards. These polyamic acids are readily dehydrated to the corresponding polyimides by heating at high temperatures, e.g. 300.degree. to 400.degree. C. These all-aromatic polyimides are not completely soluble and, therefore, protective films cannot be coated directly in the polyimide form. Instead, it is customary practice to use instead the precursor polyamic acids which have the advantage of being readily soluble in aprotic solvents. Such polyamic acid solutions are then coated to form the appropriate film and converted to the corresponding polyimide by heating the film at high temperature to effect volatilization of the solvent and dehydration of the polyamic acid to form the polyimide.
In the use of polyimide coatings as insulating layers over electronic devices, it is extremely important that the layer be as nearly planar as possible so that uniform overlying functional layers can be readily applied. As used herein, the term "planarity" refers to the ability of a material to flow over structures of various shapes and produce a planar surface. In other words, the polyimide layer should be flat and not exhibit the surface imperfections of the underlying substrate and/or electronic components. This is extremely difficult to do for the reason that, as solvent is evaporated during the conversion of the polyamic acid to the polyimide form, the coating shrinks and tends to become rigid and will not readily flow out to form a planar layer. Thus it would be highly desirable to have a coating solution in which the formation of the polyimide is delayed so that the reaction system is less viscous at the high solids level necessary to minimize film shrinkage.